Rifting Events Research Articles

Structural and geochemical evidence for a failed rift crustal evolution model in Western Dharwar Craton, South India

Detailed geological mapping along with structural and geochemical analysis were carried out in the Kibbanahalli Schist Belt (KSB) of the Bababudan Group in the Western Dharwar Craton (WDC). Repeated sequences of sedimentary rocks and volcanic rocks with conformable contact relations are the major lithological association in KSB. Sedimentary structures and field relations indicate that the Bababudan Group begins with basal conglomerate composed of debris (talus) and alluvial fan deposits or fluvial deposits. Volcanic rocks preserve features of explosive volcanism in an aqueous environment.Based on the field and microstructural observations five stages of deformations (D0, D1, D2, D3, and D4) are characterized from the study area. Among these deformation events, D2 is the major regional-scale event. East dipping NW–SE trending thrust faults and F2 parallel folds that are upright and isoclinal mark the D2 event. These F2 folds are sandwiched between D2 thrust faults. D2 structures suggest the temporal evolution of a fold-and-thrust belt by inversion of an intracratonic basin.Trace element geochemical analysis of the mafic to intermediate rocks from KSB and Chitradurga Schist Belt (CSB) shows three types of patterns. Type 1 has Nb–Ta negative anomalies, Type 2 is relatively flat trend and Type 3 has Zr–Hf positive anomalies. Type 1 has slightly enriched LREE trend while relatively flat LREE trend in Type 2 and 3. HREE trend is relatively flat in all samples, except for one sample from Type 3. The geochemical variations are related to the degree of crustal contamination experienced during their eruption in connection with the intracratonic extension.A pair of thrust faults and folded sequences of volcanic, volcanoclastic and sedimentary rocks summarize the major structural association in the study area. This association is identical to an inverted failed rift structure. Geochemical evidence also points to an intracratonic mafic to intermediate volcanism. Our study thus provides a well-preserved example of Archean failed rift. We thus propose the post-3.0 Ga crustal growth throughout the Western Dharwar Craton is therefore initiated by similar failed rifting events.

The Beni Bousera marbles, record of a Triassic-Early Jurassic hyperextended margin in the Alpujarrides-Sebtides units (Rif belt, Morocco)

The timing and process of exhumation of the subcontinental peridotites of the Gibraltar Arc (Ronda, Beni Bousera) have been discussed extensively over the last decades. In this work, we contribute to this debate through the first mapping, structural and petrological analyses, and SHRIMP U-Th-Pb dating of high-grade marbles that crop out around the Beni Bousera antiform of the Alpujarrides-Sebtides units of northern Rif (Morocco). These marbles, here termed the Beni Bousera marbles (BBMs), instead of being intercalations in the granulitic envelope (kinzigites) of the Beni Bousera peridotites, as previously described, form minor, dismembered units within a ∼30 to 300 m thick mylonitic contact between the kinzigites and the overlying gneisses of the Filali Unit (Filali–Beni Bousera Shear Zone, FBBSZ). They display silicate-rich dolomitic marbles, sandy-conglomeratic calcareous marbles and thinly bedded marble with interleaved biotite-rich schists. An unconformable contact, either of stratigraphic or tectonic origin, with the underlying kinzigites, is observed locally. Pebbles or detrital grains include K-feldspar, quartz, almandine garnet and zircon. Peak mineral assemblages consist of forsterite, Mg-Al-spinel, geikielite (MgTiO3), phlogopite and accessory zirconolite, baddeleyite and srilankite in dolomite marble, as well as K-feldspar, scapolite, diopside, titanite and accessory graphite and zircon in calcite marble. These assemblages characterize peak HT-LP metamorphic conditions close to 700–750 °C, ≤4.5 kbar. The FBBSZ includes minor ductile thrusts that determine kinzigite horses or slivers carried NW-ward over the marbles. Within the latter, NNE-trending folds are conspicuous. Brittle, northward-dipping normal faults crosscut the FBBSZ ductile structures. Detrital cores of zircon from the BBMs yield two U-Th-Pb age clusters of ∼270 Ma and ∼340 Ma, whereas their rims yield ∼21 Ma ages. Correlations with comparable settings in other West Mediterranean Alpine belts are discussed. The BBMs compare with the Triassic carbonates deposited over the crustal units of the Alpujarrides-Sebtides. The assumed Triassic protoliths may have been deposited onto the kinzigites or carried as extensional allochthons over a detachment in the Early Jurassic during the incipient formation of the Alboran Domain continental margin. Thus, it is concluded that the Beni Bousera mantle rocks were exhumed to a shallow depth during early rifting events responsible for the birth of the Maghrebian Tethys.

Silurian A-type metaquartz-syenite to -granite in the Eastern Anatolia: Implications for Late Ordovician-Silurian rifting at the northern margin of Gondwana

The Anatolide-Tauride block in the Tethyan belt represented passive northern continental margin of Gondwana throughout Paleozoic time. In this paper, we report on a relatively large amphibolite-facies metaquartz-syenite to -granite complex (~ 60 km2) in the Eastern Anatolia, the easternmost part of the Anatolide-Tauride block. Igneous textures in the metaintrusive complex are variably preserved. The preserved igneous minerals include mesoperthite + quartz ± aegirine-augite ± sodic amphibole ± calcic amphibole ± biotite and ± allanite. Mesoperthite is the sole feldspar in well-preserved igneous textures, suggesting hypersolvus nature of the original quartz syenite to granite. In the well-recrystallized samples, mesoperthite is partially or totally replaced by discrete grains of plagioclase and K-feldspar. U–Pb dating on igneous zircons from three samples yielded Silurian igneous crystallization ages (430–440 Ma). All rock types have typical geochemical characteristics of A-type syenite to granite with relatively high abundances of FeO*, Na2O, K2O, Nb, Ga, Zr, Ce, and Zn, and low abundances of MgO, CaO and Sr, resulting in high ratios of Fe/Mg and Ga/Al. Geochemical features such as (i) the absence of negative Nb–Ta anomalies on multielement variation diagrams, (ii) the enrichment of Nb and Ce relative to Y, and (iii) Y/Nb, Yb/Ta and Ce/Nb ratios similar to those in ocean island basalts point to A1-type syenite and granite which are thought to be differentiates of ocean island basaltic magmas. Estimated magma temperatures based on zirconium saturation range from 850 to 990 °C, indicating derivation of the original quartz syenite and granite from high-temperature juvenile magmas. Initial εHf values of the igneous zircons from two samples are 2.59 ± 1.14 and 2.30 ± 1.29, while δ18O values of the zircons range from 5.41 to 7.32‰ with a median of 6.50 ± 0.48‰. All these isotopic characteristics suggest that the quartz syenite to granite were derived from high-temperature mantle magmas with insignificant crustal assimilation. The Late Ordovician-Silurian A-type igneous rocks in the northern part of the Anatolide-Tauride block were probably related to a rifting event at the northern margin of Gondwana, leading to the opening of the Paleo-Tethys. This interpretation is also supported from (i) regional geological features of the Early Paleozoic sedimentary successions suggestive of extensional setting during the Late Ordovician-Silurian, (ii) the presence of Silurian anorogenic magmatism in continental blocks detached from Gondwana during Early Paleozoic time, and (iii) the presence of Silurian deep sea sedimentary blocks in Paleo-Tethyan accretionary complexes.

Zircon U–Pb Geochronology, Geochemistry and Geological Significance of the Anisian Alkaline Basalts in Gejiu District, Yunnan Province

The Gejiu Anisian alkaline basalts (GAAB), distributed in the southern part of the Emeishan large igneous province (ELIP), are crucial to understand the tectonomagmatic activity during the Triassic. Geochronological, geochemical, and Sr-Nd-Pb isotopic analyses were systematically applied to explore the origin, petrogenesis, and tectonic setting of the GAAB, and how they relate to the ELIP. Zircon U-Pb dating set the eruption date at 244 Ma. Most of the samples belonged to alkaline basalts and had high TiO2 (2.14–3.23 wt.%) and MgO (4.43–19.58 wt.%) contents. Large ion lithophile elements (LILEs) were enriched relative to high field strength elements (HFSEs). The rare earth elements (REEs) and trace element signatures in the normalized diagrams were similar to oceanic island basalts (OIB) and Emeishan high-Ti basalts. These samples had consistent Sr-Nd isotope compositions: the initial 87Sr/86Sr values ranged from 0.7044 to 0.7048 and εNd(t) = 3.25–4.92. The Pb isotopes were more complex, the (206Pb/204Pb)t, (207Pb/204Pb)t, (208Pb/204Pb)t ratios were 17.493–18.197, 15.530–15.722, and 37.713–38.853, respectively. Our results indicate that the GAAB originated from the deeper enriched mantle with 5% to 15% partial melting of garnet lherzolite and a segregation depth of 2 to 4 GPa (60–120 km). During the formation of the GAAB, clinopyroxene and Ti-Fe oxides were fractionally crystallized with insignificant crustal contamination. The GAAB were formed in a extensional regime that was related to the Gejiu-Napo rift event in the Triassic.

Strain migration during multiphase extension, Stord Basin, northern North Sea rift

AbstractIn regions experiencing multiple phases of extension, rift‐related strain can vary along and across the basin during and between each phase, and the location of maximum extension can differ between the rift phase. Despite having a general understanding of multiphase rift kinematics, it remains unclear why the rift axis migrates between extension episodes. The role pre‐existing structures play in influencing fault and basin geometries during later rifting events is also poorly understood. We study the Stord Basin, northern North Sea, a location characterised by strain migration between two rift episodes. To reveal and quantify the rift kinematics, we interpreted a dense grid of 2D seismic reflection profiles, produced time‐structure and isochore (thickness) maps, collected quantitative fault kinematic data and calculated the amount of extension (β‐factor). Our results show that the locations of basin‐bounding fault systems were controlled by pre‐existing crustal‐scale shear zones. Within the basin, Permo‐Triassic Rift Phase 1 (RP1) faults mainly developed orthogonal to the E‐W extension direction. Rift faults control the locus of syn‐RP1 deposition, whilst during the inter‐rift stage, areas of clastic wedge progradation are more important in controlling sediment thickness trends. The calculated amount of RP1 extension (β‐factor) for the Stord Basin is up to β = 1.55 (±10%, 55% extension). During the subsequent Middle Jurassic‐Early Cretaceous Rift Phase 2 (RP2), however, strain localised to the west along the present axis of the South Viking Graben, with the Stord Basin being almost completely abandoned. Rift axis migration during RP2 is interpreted to be related to changes in lithospheric strength profile, possibly related to the ultraslow extension (<1 mm/year during RP1), the long period of tectonic quiescence (ca. 50 myr) between RP1 and RP2 and possible underplating. Our results highlight the very heterogeneous nature of temporal and lateral strain migration during and between extension phases within a single rift basin.

Employing geochemistry and geochronology to unravel genesis and tectonic setting of iron oxide-apatite deposits of the Bafq-Saghand metallogenic belt, Central Iran

The Bafq-Saghand metallogenic belt is located in the central part of the Kerman–Kashmar tectonic zone and contains 39 individual occurrences of iron oxide-apatite ± REE mineralizations. These mineral concentrations, e.g., Chadormalu, Choghart, Sechahun, and Esfordi, comprise a total of ~ 1500 million tons of iron ore with an average grade of ~ 55% Fe. In terms of origin, time, and geodynamic setting, several modes of formation have been suggested for these ore deposits, including magmatic, hydrothermal, and banded iron formation scenarios. In the present study, the tectonic setting and metallogeny of iron oxide-apatites of the Bafq-Saghand belt are investigated utilizing trace element geochemistry, age dating, and oxygen isotope analyses. The geochemical characteristics of apatite and magnetite and the δ18O values of magnetite (from − 0.1 to + 2.2 ‰) indicate a dominantly magmatic-hydrothermal (δ18O > + 0.9 ‰) formation process, although primary magmatic mineralizations were locally leached and hydrothermally redeposited (e.g., samples with δ18O < + 0.9 ‰). The Cambrian volcano-sedimentary host rocks to the mineralization is intruded by calc-alkaline tonalite, trondhjemite, granodioritic, dioritic, and granitic rocks that formed in association with subduction of the Proto-Tethys Ocean under the Gondwana supercontinent in the Neoproterozoic to Early Cambrian (525–547 Ma). Additionally, a later geodynamic episode produced intrusions of alkaline syenite and monzosyenite bodies during a continental rifting event. We provide new geochronological constraints for these younger alkaline igneous rocks that document a temporal range from 421 to 447 Ma for their intrusion. In combination with the previously reported overlapping ages of the older calc-alkaline magma bodies (525–547 Ma) with the volcano-sedimentary host rock (528 Ma) and the iron oxide mineralization (510–539 Ma), we can now exclude continental rifting as a geodynamic processes that is linked to ore formation in the region. Our results corroborate that the Bafq iron ore mineralization formed during subduction of the Proto-Tethys Ocean under the Gondwana supercontinent in a near surface continental margin setting.

Unravelling the architecture and evolution of the inverted multi-stage North Iberian-Bay of Biscay rift

Current rift models propose polyphase rift evolutions that develop with the same kinematic framework and accommodate continuous and sequential extension, through the stacking of deformation modes. Several rifted margins show, however, evidence for multiple and out of sequence rift events, which developed within different kinematic frameworks, with a complex spatial and temporal evolution. In this work, we address the problem of multi-stage rifting based on the study of the central North Iberian margin, located at the southern Bay of Biscay triangular oceanic domain. This magma-poor rifted margin recorded three major Mesozoic rift events and a subsequent Alpine compressional reactivation, representing a unique setting to study the architecture of multi-stage rift systems and the exerted control on subsequent reactivation. Relying on a dense set of 2D seismic reflection profiles, boreholes, and published velocity models, we map and describe structural domains and major extensional and compressional structures, and we construct depth and thickness maps of syn-rift units. These new maps display the geometry and spatial distribution of major rift basins and bounding structures. Distinctive scenarios resulting from this tectono-stratigraphic approach led us to define three rift systems. A first, diffuse and widespread Triassic system, with classical fault-bounded half-graben basins; a second, narrow, deep and localised Late Jurassic to Barremian transtensional system, including laterally confined pull-apart basins; and a third, widely distributed, Aptian to Cenomanian hyperextended system. Our results show that each rift system controlled the successive extensional events and the subsequent contractional deformation, resulting in a complex 3D template. Deciphering the tectono-stratigraphic architecture of such, multi-stage systems provide key insights on the spatial, temporal, and thermal evolution of divergent plate boundaries. It is likewise indispensable to propose and test kinematic plate deformable models on conjugate rifted margins and to comprehend the implications for their reactivation.

Volcanic rocks in Lower Jurassic marine carbonate successions in the southeastern Iberian Range (Spain): biochronostratigraphic dating

The Ligurian Tethys began to open in a roughly NE–SW direction in Pliensbachian times. West of the Ligurian oceanic crust, a carbonate-platform system constituted the eastern Iberian paleomargin. Lower and Middle Jurassic marine sediments deposited in these shallow platforms are well exposed today in the southeastern Iberian Range and host multiple volcanogenic deposits, mainly volcaniclastic in nature. Regional mapping reveals that volcanic outcrops show NW and NE (or NNE) oriented lineaments following tracks of cortical weakness. Volcanic ages were estimated by using biochronostratigraphic methods and sequence-stratigraphic correlations. As a result, ten Lower Jurassic volcanic episodes have been identified. The chronostratigraphic positions of the volcanic deposits span from the early Pliensbachian (Jamesoni Zone or later) to the late Toarcian (Thouarsense Zone). The extension-dominated regime of the eastern Iberian paleomargin also affected the southern Iberian paleomargin (Betic Cordillera) as a result of the eastward prolongation of the central Pangea fragmentation. The break-up of wide platforms and the first volcanic emissions seem to be roughly coeval in the Betic and Iberian domains and synchronous with the onset of rifting and spreading events of the Ligurian Tethys Ocean.

Pre‐breakup Extension in the Northern North Sea Defined by Complex Strain Partitioning and Heterogeneous Extension Rates

AbstractThe early stages of continental rifting are accommodated by the growth of upper crustal normal fault systems that are distributed relatively evenly across the rift width. Numerous fault systems define fault arrays, the kinematics of which are poorly understood due to a lack of regional studies drawing on high‐quality subsurface data. Here we investigate the long‐term (~150 Myr) growth of a rift‐related fault array in the East Shetland Basin, northern North Sea, using a regionally extensive subsurface data set comprising 2‐D and 3‐D seismic reflection surveys and 107 boreholes. We show that rift‐related strain during the pre‐Triassic to Middle Triassic was originally distributed across several subbasins. The Middle to Late Triassic saw a decrease in extension rate (~14 m/Myr) as strain localized in the western part of the basin. Early Jurassic strain initially migrated eastward, before becoming more diffuse during the main, Middle‐to‐Late Jurassic rift phase. The highest extension rates (~89 m/Myr) corresponded with the main rift event in the East Shetland Basin, before focusing of strain within the rift axis and ultimate abandonment of the East Shetland Basin in the Early Cretaceous. We also demonstrate marked spatial variations in timing and magnitude of slip along strike of major fault systems during this protracted rift event. Our results imply that strain migration patterns and extension rates during the initial, prebreakup phase of continental rifting may be more complex than previously thought; this reflects temporal and spatial changes in both thermal and mechanical properties of the lithosphere, in addition to varying extension rates.

Detrital zircon U-Pb and Lu-Hf data for a kinzigitic gneiss (Jequitinhonha Complex, Araçuaí Orogen, SE Brazil) constrain the age of a huge storage of Ediacaran carbon

The precursor basin of the Araçuaí – West Congo Orogenic System (AWCO) formed a large gulf within the São Francisco – Congo Paleocontinent (SFCP), where sedimentary successions ascribed to the Jequitinhonha Complex and Macaúbas Group were deposited in the Neoproterozoic. This precursor basin system comprises an aborted continental rift of Early Tonian age and a Cryogenian rift that evolved to ocean-floor spreading up to the Early Ediacaran. The Jequitinhonha Complex is an extensive metasedimentary unit located in the northeastern Araçuaí Orogen, consisting of Al-rich paragneiss (kinzigite), sillimanite-graphite gneiss and associated economic deposits of flake graphite, quartzite and lenses of calc-silicate rock, metamorphosed to upper amphibolite or granulite facies around 570–550 Ma. U-Pb (LA-MC-ICP-MS) data of 110 detrital zircon grains from the paleosome of a migmatitic paragneiss yield five main age peaks at 2564 ± 35 Ma (5% of the grains), 2012 ± 21 Ma (7%), 1807 ± 19 Ma (12%), 960 ± 8 Ma (29%) and 662 ± 6 Ma (48%), with the youngest grains around 630–609 Ma, and a major age gap from the Statherian to Stenian. The U-Pb age spectrum coupled with Lu-Hf in zircon (LA-MC-ICP-MS) data for 106 zircon grains suggest the following sources: Neoarchean to Orosirian grains were sourced from the SFCP basement, Early Tonian grains from the anorogenic magmatic province, and Cryogenian to Early Ediacaran grains were provided by the Southern Bahia Alkaline Province and Rio Doce magmatic arc. The εHf(t) values show a predominant contribution from evolved crust, with minor involvement of moderately juvenile crust. The sedimentation of Jequitinhonha Complex protoliths developed from the Cryogenian to Early Ediacaran, partially within anoxic marine environments favorable to the preservation of organic matter that formed the huge graphite deposits and lots of smaller occurrences found in the region, accumulating probably more than one billion tons of raw graphite. The youngest detrital zircon population suggests that the deposition record of the Jequitinhonha Complex covers the Cryogenian continental rifting event followed by the development of oceanic crust to the onset of subduction and building of the Rio Doce magmatic arc in the Early Ediacaran. The regional accumulation and preservation of carbon-rich sediments (now graphite-rich gneiss) most probably represent one of the largest storages of Ediacaran carbon around the world.

Long Live the Laurentian Great Lakes

Living in Geologic Time: Billion-year-old rifting events set the stage for Earth’s greatest lakes.

Play distribution and the hydrocarbon potential of the Mannar Basin, Sri Lanka

The Mannar Basin is a frontier failed rift basin between India and Sri Lanka. The Sri Lankan part has an area exceeding 42,000 km2. Although the recent two gas discoveries have confirmed the existence of an active petroleum system in the Mannar Basin, a major portion of the basin is still poorly explored. This article summarized the progress of current exploration activities and the hydrocarbon potential of the Mannar Basin. This basin began to evolve since the Upper Jurassic and experienced two rifting events; an early Late Jurassic syn-rift phase associated with East–West Gondwana break up; and a later, earliest Cretaceous syn-rift phase associated with Antarctica separation from greater India around 142 Ma. Rifting was followed by a post-rift phase comprising a thermal sag period and an inversion period. Three potential source rocks intervals have been interpreted at Maastrichtian–Campanian, Albian–Aptian, and Late Jurassic stratigraphic levels. The basin modelling work has confirmed that (1) mature potential source rocks (mainly Type II) exist below the Maastrichtian–Campanian strata and (2) the best potential source rocks (mainly Type II) exist at Albian–Aptian stratigraphic levels. The Late Jurassic source rocks have more potential for gas, while other sources have potential for both oil and gas. According to basin modelling results, Maastrichtian–Campanian and Albian–Aptian source rocks reach the oil window in the present-day depocentre around 45 Ma and 80 Ma, respectively. The Late Jurassic source rocks (mainly Type III) reach the gas window around 112 Ma in the present-day depocentre. Five play levels were defined for the whole stratigraphic section of the Mannar Basin. Tertiary play level is dominated by submarine fans, mounds and rollover anticline like structures. The Upper Cretaceous play is dominated by forced-fold structures, intra-basalt turbidite sands, and sub-volcanic sand-rich systems. The Lower Cretaceous play is dominated by reefs and abrupt margin pinch outs. The Upper Jurassic play is dominated by abrupt margin pinch outs. The Basement play consists of weathered basement rocks. The main challenge of the Mannar Basin is imaging below the flood volcanic layer, which inhibits the penetration of seismic energy and results in low-quality seismic data. Therefore, hydrocarbon potential assessments have become a major challenge below the Upper Cretaceous. The interpretation shows that the basin has a low risk for the source and reservoir, and high risk for seal and traps. New exploration activity would unlock more potential areas for hydrocarbon accumulations. Finally, the findings of this study can help for better understanding of hydrocarbon potential areas and current progress of exploration activities in the Mannar Basin, Sri Lanka.

Triassic magmatism in the European Southern Alps as an early phase of Pangea break-up

AbstractMagmatic rocks from the Dolomites, Carnic and Julian Alps, Italy, have been sampled to investigate the origin and geodynamic setting of Triassic magmatism in the Southern Alps. Basaltic, gabbroic and lamprophyric samples have been characterized for their petrography, mineral chemistry, whole-rock major and trace elements, and Sr, Nd and Pb isotopic compositions. Geothermobarometric estimates suggest that the basaltic magmas crystallized mostly at depths of 14–20 km. Isotopic data show variable degrees of crustal contamination decreasing westwards, probably reflecting a progressively more restitic nature of the crust, which has been variably affected by melting during the Permian period. Geochemical and isotopic data suggest that the mantle source was metasomatized by slab-derived fluids. In agreement with previous studies and based on geological evidence, we argue that this metasomatism was not contemporaneous with the Ladinian–Carnian magmatism but was related to previous subduction episodes. The lamprophyres, which likely originated some 20 Ma later by lower degrees of melting and at higher pressures with respect to the basaltic suite, suggest that the mantle source regions of Triassic magmatism in the Dolomites was both laterally and vertically heterogeneous. We conclude that the orogenic signatures of the magmas do not imply any coeval subduction in the surrounding of Adria. We rather suggest that this magmatism is related to the Triassic rifting episodes that affected the western Mediterranean region and that were ultimately connected to the rifting events that caused the break-up of Pangea during the Late Triassic – Early Jurassic period.

Cretaceous lithostratigraphy of North-East Greenland

An updated and revised lithostratigraphic scheme is presented for the Cretaceous of North-East Greenland from Traill Ø in the south to Store Koldewey in the north. The Ryazanian to lower Maastrichtian succession is up to several kilometres thick and comprises four groups, 12 formations and 18 members. The groups record the tectonic evolution of the East Greenland depocentre on the western flank of the evolving proto-Atlantic seaway. The Wollaston Forland Group encompasses the uppermost Jurassic – lowermost Cretaceous rift-climax succession and contains the Lindemans Bugt and Palnatokes Bjerg Formations; two new members of the latter formation are erected from Store Koldewey. Post-rift Cretaceous strata are referred to the new Brorson Halvø Group and the Home Forland Group. The Brorson Halvø Group (uppermost Hauterivian – middle Albian) is dominated by slope and basinal mudstones of the new Stratumbjerg Formation but also includes fluvio-deltaic and shallow marine sandstones of the revised Steensby Bjerg Formation on northern Hold with Hope and submarine slope apron breccias and conglomerates of the revised Rold Bjerge Formation on Traill Ø. The Home Forland Group covers the middle Albian – Coniacian succession. The basal unconformity records an important mid-Albian tectonic event involving intrabasinal uplift, tilting and erosion, as exemplified by the middle Albian conglomerates of the new Kontaktravine Formation on Clavering Ø. The Home Forland Group is dominated regionally by mud-dominated slope to basinal deposits of the elevated and revised Fosdalen Formation; it also includes lowstand basin-floor fan sandstones of the new upper Albian Langsiden Member. The new Jackson Ø Group (upper Turonian – lower Maastrichtian), records a phase of basin reorganisation marked by a significant fall in sedimentation rate in North-East Greenland, probably linked to rift events in, and bypass to, the central proto-Atlantic rift system. The base of the group is an erosional unconformity on Traill Ø and Geographical Society Ø overlain by submarine slope-apron conglomerates of the Turonian Månedal Formation. The base is conformable on Hold with Hope but is defined by a condensed interval (the Coniacian Nanok Member) that is succeeded conformably by slope and basin-floor turbidite sandstones of the Coniacian–Santonian Østersletten Formation and slope to basinal mudstones of the Campanian – lower Maastrichtian Knudshoved Formation. The new Leitch Bjerg Formation of Campanian slope-apron conglomerates and sandstones in eastern Geographical Society Ø erosionally overlies the Knudshoved Formation.

The role of sub-continental mantle as both “sink” and “source” in deep Earth volatile cycles

The extent to which Earth’s sub-continental lithospheric mantle modulates the flux of volatile elements from our planet’s deep interior to its atmosphere (via volcanism) is poorly constrained. Here, we focus on “off-craton” sub-continental lithospheric mantle because this long-lived reservoir potentially acts as both a volatile “sink” and “source” during major heating and rifting events. The sub-continental lithospheric mantle is primarily formed of peridotites with subordinate amounts of pyroxenites. While both lithologies are dominated by nominally-volatile-free mantle minerals, some of these phases have been shown to contain non-negligible amounts of H2O (e.g. 100’s of ppmw in clinopyroxene). Data for volatile elements other than Li are, however, limited. We present new, high-precision, in-situ Secondary Ion Mass Spectrometry analyses of H, F, Cl, Li and B in olivine and pyroxenes from well-characterised garnet- and spinel-bearing peridotites and pyroxenites (from southern Patagonia and the Antarctic Peninsula). Our study confirms that clinopyroxene is the main host of H2O and F. The maximum F contents we report (up to 154 ppmw) are higher than those in previous studies and occur in Ti-Cr diopsides in highly-metasomatised peridotites and Ti-Al augites from clinopyroxenite veins. Water contents of clinopyroxenes (up to 615 ppmw) are within the range previously published for continental mantle. Lithium concentrations are low (<5 ppmw) in all analysed phases and both Cl and B are below detection levels (14 ppmw and 0.03 ppmw, respectively).Unique to our study is the large variation in major- and trace-element concentrations of the clinopyroxenes, which allows us to place quantitative constraints on how volatiles are stored in the mantle. We demonstrate that: (i) F contents of clinopyroxenes closely correlate with Ti and (ii) DHCpx-Opx and DFCpx-Opx is systematic and inversely correlated with temperature. Despite the redistribution of volatiles during sub-solidus re-equilibration, we show that the first order control on the concentration of volatiles in clinopyroxene is the style of metasomatism, i.e. channellised flow versus reactive percolation. The mean bulk volatile contents of peridotites from Pali Aike and the Antarctic Peninsula (H2O = 89 ± 31 ppmw, F = 16 ± 11.2 ppmw and Li = 2 ± 0.7 ppmw) are within the range previously published for continental “off-craton” mantle. The pyroxenites have significantly higher mean bulk concentrations of H2O (260 ± 59 ppmw), F (86 ± 43 ppmw) and Li (1.0 ± 0.35 ppmw). While the greater capacity of mantle pyroxenites to host H2O relative to the associated peridotites has previously been observed in global “off-craton” mantle xenolith suites (e.g. Oahu, Hawaii; eastern China and the Rio Grande Rift, SW USA), here we show for the first time that pyroxenites are also major hosts of F (but not Cl, Li or B). Because of their relatively low solidus temperatures, pyroxenites in “off-craton” settings will be readily re-mobilised during lithospheric extension (and heating). We suggest these pyroxene-rich mantle lithologies may be responsible for the elevated concentrations of H2O and F observed in basalts and volcanic gasses from major continental rift zones and flood basalt provinces, and hence an important consideration in models of global volatile cycles.

Tectonic implications of the gravity signatures of the Midcontinent Rift and Grenville Front

North America's Midcontinent Rift (MCR) and Grenville Front (GF) jointly record aspects of the complex history of the assembly of Rodinia. The ~1100 Ma MCR, remaining from a failed major rifting event, is exposed along Lake Superior and well defined by gravity, magnetic, and seismic data. The GF, which results from collisions with Laurentia, is exposed in and identified by seismic and potential field data in Canada. In the eastern U.S., lineated gravity highs extending southward from Michigan to Alabama, along the trend of the front in Canada, have been interpreted either as a buried Grenville Front or as part of the MCR's east arm. We explore this issue by examining the gravity signatures of the MCR and GF. Both the MCR's arms have pronounced gravity highs, with the west arm's greater than the east arm's. Combining the gravity observations with seismic data suggests that the west arm contains 20–25 km thickness of volcanics, whereas the east arm contains 10–15 km of volcanics. Along the Grenville Front in Canada, thickened crust along the northern portion causes a broad gravity low, whereas the stacked thrusts along the southern portion cause essentially no gravity signature. Hence the lineated gravity highs in the eastern U.S. appear similar to those along the remainder of the MCR, and unlike those on either portion of the GF. These data favor the gravity anomalies traditionally interpreted as the Grenville Front in the eastern U.S. as instead being part of the MCR's east arm. A thrust sheet structure like that of the southern Canadian Grenville Front - which would have essentially no gravity effect - could also be present along the MCR's east arm, as implied by recent EarthScope seismic data.

Ferroan alkalic volcanism associated with Calymmian rifting in the Paramirim aulacogen, São Francisco craton, Brazil: New insights from lithofacies analysis and evidence of mantle-derived alkaline H2O-rich metasomatic fluids affecting ancient crustal materials

Rift-related volcanism is widely used in models of supercontinent reconstruction, while the associated metasomatic fluids provide information about the mantle-crust process during intracrustal melting events. The Paleoproterozoic/Mesoproterozoic Paramirim aulacogen corresponds to multiple intracratonic rift cycles that register many basin developing events. Intense volcanic activity is recorded in some rift events, but the volcanic lithofacies and their petrogenesis are poorly constrained. Herein, we present lithofaciological characterization, whole-rock and mineral chemistry, and U-Pb and Lu-Hf isotope data from the 1.58 Ga volcanic rocks. Trachytic and trachy-andesitic lava flows and primary to secondary volcaniclastic lithotypes characterize this volcanism. The primary volcaniclastic rocks resulted from direct volcanic activity and can be divided into pyroclastic and hydroclastic lithofacies associations. The pyroclastic lithotypes correspond to air-fall, density currents and block-and-ash flow deposits formed in explosive eruptions; hydroclastic lithotypes are represented by hyaloclastites and peperites, reflecting effusive eruptions and interactions between magma, water and sediments. The secondary volcaniclastic lithotypes resulted from late-stage reworking processes and consist of debris flows, ephemeral streams associated with high energy and gravitational flow deposits. The lava flows show well-preserved primary volcanic features, such as magma flow textures and euhedral phenocrysts. Based on petrographic and chemical characteristics, associated with negative ɛHf(t) values, we infer that high melt rate of an ancient crust, previously metasomatized by H2O-rich alkaline mantle-derived fluids, generated the alkalic ferroan lavas. These fluids promoted potassic and sodic metasomatism that affected all the volcanic rocks, modifying the chemistry of the feldspars to pure orthoclase (Or96-98) and albite (Ab98-100). In some lavas, a higher concentration of Na-rich fluids synthesized aegirine at a minimum temperature of 200 °C and promoted its peralkaline character. The potassic-sodic metasomatism influenced the volcano-sedimentary sequence of the Pajeú Group and the oldest rocks of the basement but did not affect the overlying sedimentary sequences, indicating that fluids ceased with the end of volcanism. We suggest that the H2O-rich alkaline fluids arose by focused degassing in response to a rising asthenospheric mantle during the Calymmian rift of the Paramirim aulacogen.

How did the peripheral subduction drive the Rodinia breakup: Constraints from the Neoproterozoic tectonic process in the northern Tarim Craton

The peripheral or exterior subduction is known to drive the breakup of Rodinia; however, the geodynamic driving mechanism remains controversial, which may result from the slab retreat or the slab avalanche-induced superplume impingement. We report here new constraints on the Neoproterozoic tectonic process whether the northern Tarim Craton at the NW margin of Rodinia was subduction- or mantle plume-dominated that further sheds light on how the peripheral subduction drove the Rodinia breakup. A tectonic transition from the Cryogenian arc and backarc rift to the Ediacaran passive margin characterized northern Tarim, as indicated by the distinct detrital zircon geochronological, geochemical, and structural-sedimentary features between the Ediacaran and Cryogenian sequences. This rejects the mantle plume-dominated evolution, but supports a Neoproterozoic evolving subduction process at the northern Tarim margin, where an advancing subduction (830–780 Ma) evolved into a retreating subduction (<780 Ma) in response to the retreat of the subducting Pan-Rodinia oceanic slab. The retreat-induced backarc rifting resulted in the opening of the northern Tarim rift between 760 and 740 Ma and the Cryogenian to Ediacaran tectonic transition. The peripheral subduction along the northern Tarim margin had thus caused no rifting before 780 Ma, which was also impossible to induce the >780 Ma rifting events in the supercontinent interior. These findings imply a composite peripheral subduction-driving mechanism for the Rodinia breakup. The slab retreat probably occurred as the dominant breakup driving force at the supercontinent margins, with less impact to the supercontinent interior; however, another geodynamic driver such as the deep subduction-induced plume impingement may account for the interior rifting and breakup. In addition, a zircon U-Pb age of 636 ± 2 Ma obtained from tufflava constrains the Tereeken diamictite to correlate with the Marinoan glaciation that further indicates a rapid and globally synchronous termination of the Neoproterozoic “Snowball Earth”.

Physical modelling of superimposed tectonic events in the Volta Redonda Basin, Southeastern Brazil

Abstract This paper presents the results of a new approach based on sandbox models to the understanding of the tectonic evolution of the Volta Redonda Basin, southeastern Brazil. Those models were designed to replicate the overlapping process of two of the four superimposed tectonic events identified in the Volta Redonda Basin: the first one, a rifting event with NW-SE extension direction that created the basin, and the subsequent E-W sinistral transcurrent tectonics. The boundary conditions for the models were set from the geological premises described in previous works, which resulted in the current conceptual geological models of the study area. The sandbox models were composed of alternating layers of colored sand, deposited on two pieces of acetate or cardboard sheets, which acted as pre-existing basement structures. Once completed, the models were cut and photographed to allow observation and interpretation of the resulting internal structures. Moreover, the interpretations maintain similarities with adopted conceptual models and permit conclusions about structures previously identified in other works. Finally, the consequent structural framework exposes the effects of structural reactivation and Basin geometry changes due to the superposition of deformational events and matches with the general structural aspects identified in field studies.

Detrital zircon records of the Paleo-Mesoproterozoic rift-sag Tamanduá Group in its type-section, Northern Quadrilátero Ferrífero, Minas Gerais, Brazil

Abstract The Quadrilatero Ferrifero metallogenetic province is located in the southernmost portion of the Sao Francisco craton, SE Brazil. The Tamandua and Cambotas ridges stand out topographically in the northeastern portion of Quadrilatero Ferrifero and show NE-SW and N-S directions, respectively. Those ridges involve metasedimentary rocks of the Tamandua Group bounded by a fault system. Due to stratigraphic and structural complexities, there is little consensus about the maximum sedimentation age and the stratigraphic position in which Tamandua Group sediments were deposited. In this work, we took advantage of the excellent exposures in the Tamandua and Cambotas ridges to present detailed stratigraphic observations combined with U-Pb zircon geochronological data from samples of different stratigraphic levels of Tamandua Group. Furthermore, we provide U-Pb data from samples of the intrusive Pedra Formosa Suite that cut the whole Tamandua sequence in the study area. Our observations showed that the Tamandua Group represents a rift-sag basin-fill succession developed along the eastern border of the Sao Francisco paleoplate. The basal metaconglomerate and metasandstone package grades upward into marine metasandstone and phyllite. Detrital zircon obtained from the basal unit, Antonio dos Santos Formation, reveals maximum depositional ages between ca. 1981 and 1770 Ma. The upper succession, Cambotas Formation, shows a maximum depositional age from 1769 to 1740 Ma. The Pedra Formosa Suite shows zircons that crystallized at ca. 1740 Ma. The stratigraphic framework and the Orosirian-Statherian ages suggest a correlation with the first rifting event within the Sao Francisco paleoplate, the precursor of Paleo- to Mesoproterozoic Espinhaco basin.